A coated type magnetic recording medium prepared by coating a magnetic coating composition comprised of ferromagnetic particles dispersed in an organic binder has widely been used as a magnetic recording medium. However, these coated type magnetic recording mediums have some defects such as low saturation magnetization due to the use of metal oxide particles as ferromagnetic particles, and the densities of ferromagnetic particles in a magnetic layer cannot be increased because the magnetic layer contains an organic binder which renders the coated type magnetic recording medium unsuitable for high density recording and manufacturing steps are complicated.
With the increased demand for higher density recording in recent years, a magnetic recording medium of a thin metal film type provided on a non-magnetic support has been developed. The thin metal film type magnetic recording medium is prepared by forming a thin metal film on a non-magnetic support by a vapour deposition method such as vacuum deposition, sputtering or ion plating or a metal plating method such as electroplating or electroless plating. Although the thin films of the magnetic compositions are not limited to metals, metals are the typical example of the magnetic compositions. Therefore, the magnetic recording medium having a thin film is hereinafter referred to as a magnetic recording medium of a thin metal film type. Further this type medium is called a magnetic recording medium of a non-binder type because the medium does not contain an organic binder.
Such a recording medium of a thin metal film type has drawn much attention recently; since the thin film is composed of ferromagnetic metals without an organic binder, a higher saturation magnetization and coercive force can be obtained as well as a reduction in film thickness as compared to the coated type magnetic recording mediums. Also, the thin metal film type medium exhibits a low effect of demagnetization at a short wavelength region and manufacturing steps can be simplified.
The thin metal film type recording medium appears to have a uniform and smooth metal surface, but upon microscopic analysis it is seen that it has such a rough structure because of the alignment of the fine metal particles. Therefore, the thin metal film type magnetic recording medium easily corrodes and has poorer weather resistance and poorer anti-corrosive properties compared to the coated type magnetic recording medium.
A magnetic recording medium is generally rubbed by a magnetic head on the surface thereof upon recording and reproduction, and when even slight corrosion is present on a metal thin film, it comes off upon friction, causing head-clogging, thereby damaging the magnetic head and the magnetic recording medium itself.
The magnetic recording medium of a thin metal film type poor durability because, as it has poor durability because, as it has a smooth surface of a thin metal film, friction is high, thereby readily causing stick-slip. The durability on a VTR of the thin metal film type medium is inferior to the coated type medium.
Known methods of attempting to improve both the weather resistance an durability of the thin metal film type magnetic recording medium include a method of nitrogenating the surface of a film layer by ion plating (Japanese Patent Application (OPI) No. 33806/75), a method of providing a silicon nitride film by sputtering (Japanese Patent Application (OPI) No. 30304/78), a method of providing a nonmagnetic surface layer by exposing a magnetic film to discharge in an atmosphere of nitrogen gas and the like (Japanese Patent Application (OPI) No. 85403/78) and a method of providing a nitrogenated thin metal film on a magnetic thin metal film (Japanese Patent Application (OPI) No. 143111/79). The term "OPI" as used herein refers to a "published unexamined Japanese patent application".
Additionally, a magnetic thin film composed of iron nitride, or of iron and iron nitride as disclosed in European Patent No. 8328 and Japanese Patent Application (OPI) No. 87809/84 has been proposed as a magnetic recording medium of a non-binder type having excellent weather resistance. Another magnetic recording medium is disclosed in Japanese Patent Application (OPI) No. 54023/86, which has a magnetic thin film mainly composed of iron oxide nitride on a non-magnetic .support. The magnetic thin film has the composition having the following formula. EQU Fe.sub.1-x-y N.sub.x O.sub.y (0.25.ltoreq.x+y&lt;0.6)
The magnetic recording medium having a thin magnetic layer composed of iron oxide nitride is hereinafter referred to as a magnetic recording medium of an iron oxide nitride thin film type.
As described above, various protective layers are provided, but further improvements as to weather resistance and durability are still necessary. That is, in order to obtain sufficient effects of a protective layer, the film thickness must be increased, resulting in decrease of outputs due to spacing loss. In the above described magnetic recording medium mainly composed of iron nitride or of iron oxide nitride, weather resistance is greatly improved, but electromagnetic characteristics are deteriorated.